Feedback Amplifier MCQ Quiz - Objective Question with Answer for Feedback Amplifier - Download Free PDF

Concept of Negative Feedback Amplifier:

Negative feedback is a technique where a portion of the output signal is fed back to the input with opposite phase to improve amplifier performance. Key effects include:

• Stabilization of gain

• Reduction of distortion and noise

• Control of input/output impedances​

Additional Information

1) Reduces nonlinear distortion in the output

• True: Negative feedback linearizes the amplifier's response, reducing harmonic and intermodulation distortion.

2) Reduces the effect of temperature on the output

• True: By stabilizing the gain, negative feedback makes the amplifier less sensitive to temperature variations in components (e.g., transistor β shifts).

3) Reduces unwanted electrical signals (noise) generated in the circuit

• True: Feedback suppresses internally generated noise (e.g., thermal noise, hum) by the same factor as distortion.

4) Reduces the bandwidth of the amplifier

• False (Correct Answer):

  • Negative feedback increases bandwidth by trading gain for frequency response.

  • The gain-bandwidth product (GBW) remains constant: Lower midband gain → Higher cutoff frequency.

Concept

A sinusoidal oscillator is an electronic circuit that produces a continuous, oscillating output signal without an external input signal. The circuit uses positive feedback to sustain the oscillations.

For a sinusoidal oscillator to operate at its frequency of oscillation, certain conditions must be met. One of the key characteristics is that the amplitude of the output signal remains constant over time.

 Additional Information

Let's discuss why the other options are incorrect:

• 1) The phase shift around the loop is maximum.
  This statement is incorrect. For a sinusoidal oscillator, the total phase shift around the feedback loop must be 360 degrees (or 0 degrees) to satisfy the Barkhausen criterion for sustained oscillations.
• 3) The loop gain is less than one.
  This statement is incorrect. For a sinusoidal oscillator to sustain oscillations, the loop gain must be equal to or slightly greater than one. If the loop gain is less than one, the oscillations will decay over time.
• 4) The frequency of the output signal varies with time.
  This statement is incorrect. For a sinusoidal oscillator, the frequency of the output signal should remain constant over time. Any variation in frequency would indicate instability or external influence.

The correct option is 4

Concept:

A. Hartley oscillator - II. LC tuned circuit

Explanation: The Hartley oscillator is an electronic oscillator circuit in which the oscillation frequency is determined by an LC (inductor-capacitor) tank circuit. The frequency can be adjusted based on the values of the inductors and capacitors used.

B. Crystal oscillator - III. Greater stability

Explanation: A Crystal oscillator uses a quartz crystal for frequency control and offers excellent frequency stability due to the quartz crystal's high Q-factor. This makes a crystal oscillator more stable compared to the other oscillator circuits.

C. Wien bridge oscillator - I. Two-stage RC coupled amplifier

Explanation: The Wien Bridge Oscillator employs a feedback circuit with an RC (resistor-capacitor) network to produce sinusoidal oscillations. Its design can involve a two-stage RC coupled amplifier and it's often used for generating audio frequencies.

The correct option is 1

Concept:

Miller integrator

An operational amplifier circuit, typically used as a square-to-sawtooth converter, in which a capacitor is connected as part of a negative-feedback loop across the input and output terminals of the amplifier, and whose output voltage is proportional to the time integral of the input voltage.

Concept: 

For the negative feedback amplifier, the closed-loop gain (A) is given by:

\(A=\frac{{{A_{OL}}}}{{1 + {A_{OL}}β }}\)

For the positive feedback amplifier, the closed-loop gain (A) is given by:

\(A=\frac{{{A_{OL}}}}{{1 - {A_{OL}}β }}\)

Where AOL is the open-loop gain

β is the gain of the feedback

Calculation:

AOL = 3000

β = 0.01

\(A=\frac{3000}{1+3000 \times 0.01}=\frac{3000}{31}=97\)

Hence the voltage gain of the amplifier with negative feedback is  97

• The transistor amplifier in which collector current flows for the entire cycle of input AC signal is called class A amplifier.
• The transistor amplifier in which collector current flows for the half-cycle of an AC signal is called a class B amplifier.
• The transistor amplifier in which collector current flows for less than half the cycle of an AC signal is called a class C amplifier

The correct option is 4

Concept:

A. Hartley oscillator - II. LC tuned circuit

Explanation: The Hartley oscillator is an electronic oscillator circuit in which the oscillation frequency is determined by an LC (inductor-capacitor) tank circuit. The frequency can be adjusted based on the values of the inductors and capacitors used.

B. Crystal oscillator - III. Greater stability

Explanation: A Crystal oscillator uses a quartz crystal for frequency control and offers excellent frequency stability due to the quartz crystal's high Q-factor. This makes a crystal oscillator more stable compared to the other oscillator circuits.

C. Wien bridge oscillator - I. Two-stage RC coupled amplifier

Explanation: The Wien Bridge Oscillator employs a feedback circuit with an RC (resistor-capacitor) network to produce sinusoidal oscillations. Its design can involve a two-stage RC coupled amplifier and it's often used for generating audio frequencies.

Negative feedback amplifier:

A negative-feedback amplifier is an electronic amplifier that subtracts a fraction of its output from its input.

The transfer function of a negative-feedback amplifier is given by:

\({V_o(s)\over V_i(s)} = {A_{ol}\over 1+\beta A_{ol}}\)

Effects of negative feedback:

1. Increases input resistance
2. Reduces gain
3. Increases bandwidth
4. Stability increases hence providing a more linear operation
5. Reduces noise and distortion

The four basic feedback topologies are as shown:

i) Voltage-series feedback with voltage amplifier

ii) Current-series feedback with a transconductance amplifier

iii) Current-shunt feedback with current amplifier

iv) Voltage-shunt feedback with trans resistance amplifier

The four basic feedback topologies are as shown:

i) Voltage-Series feedback with voltage amplifier:

Input Resistance:

\({R_{if}} = {R_i}\left( {1 + A\beta } \right)\) (Increases)

Output Resistance:

\({R_{of}} = \frac{{{R_o}}}{{1 + A\beta }}\) (Decreases)

ii) Current-Series feedback with a transconductance amplifier:

Input Resistance:

\({R_{if}} = {R_i}\left( {1 + A\beta } \right)\)  (Increases)

Output Resistance:

\({R_{0f}} = {R_0}\left( {1 + A\beta } \right)\) (Increases)

iii) Current-Shunt feedback with a current amplifier:

Input Resistance:

\({R_{if}} = \frac{{{R_i}}}{{1 + A\beta }}\)  (Decreases)

Output Resistance:

\({R_{of}} = {R_o}\left( {1 + A\beta } \right)\) (Increases)

iv) Voltage-Shunt feedback with trans resistance amplifier:

Input Resistance:

\({R_{if}} = \frac{{{R_i}}}{{1 + A\beta }}\) (Decreases)

Output Resistance:

\({R_{of}} = \frac{{{R_o}}}{{1 + A\beta }}\) (Decreases)

The net input resistance for the current shunt is given by:

\({{\rm{R}}_{{\rm{iF}}}} = \frac{{{{\rm{R}}_{\rm{i}}}}}{{1 + {\rm{A\beta }}}}\)

And the output resistance is:

ROF = Ro (1 + Aβ)

Conclusion: The effect of current-shunt Feedback in an amplifier is to decrease the Input resistance and increase the output resistance.

Negative feedback circuit:

The feedback amplification factor is given by:

 \({A_f} = \frac{A}{{1 + A\beta }}\),

where,

A is open-loop gain

Aβ is the loop gain.

The negative feedback in amplifiers causes:

• Reduced the gain and increases the stability in gain
• Increases the bandwidth to maintain constant gain-bandwidth product
• Reduces the distortion and noise in the amplifier
• The signal to noise ratio is not affected.

Concept-

Identification of Feedback topology-

1. Identify the feedback network/element.

2. If at the output side, feedback is connected to the output of the circuit directly, name it as 'voltage sampling', or else 'current sampling'

3. If at the input side, feedback is connected to the input given to the circuit directly, name it as 'shunt mixing ' or else 'series mixing'

Analysis:

Step-1: feedback element is both R₁­ Resistor

Step – 2: The feedback element is directly connected to output so voltage sampling.

Step – 3: The feedback element is directly connected to input so shunt mixing.

Hence, Voltage-shunt is the right answer. 

Transistor as an Amplifier

A transistor is a 3-terminal device i.e. base, collector, and emitter 

When the base-emitter junction is forward-biased and the collector-emitter is reversed-biased, the transistor acts as an amplifier.

The working of the amplifier is based on negative feedback.

In a negative feedback circuit, some part of the output is fed back to the input.

Explanation

The closed loop gain for negative feedback is given by:

\(A_f={A\over 1+AM}\)

where, Af = Closed loop gain

A = Open loop gain

M = Feedback factor

So, when negative feedback amplifier, the gain reduces by the factor (1+AM)

Concept:

There are 4 possible combinations for Voltage and Current with which we can sample at the output and mix the feedback to the input.

 Sampling:

• On the output side, we will take a sample of output since we want to check the behavior of output and we don't want to disturb the output when we take the sample.
• That's why when the voltage is sampled, it is in parallel (as the voltage is the same in parallel) and current in series (as the current is the same in series).

Mixing:

• In the mixing end, we want to affect the signal that is provided to the amplifier since that is the actual fundament of giving the feedback.
• So, the voltage will be in series and the current will be in parallel. So that they can change the input and effect the change in output.

The four basic feedback topologies are as shown:

i) Voltage-sampling voltage-mixing (series-shunt) topology.

ii) current-sampling voltage-mixing (series-series) topology.

iii) current-sampling current-mixing (shunt-series) topology.

iv) voltage-sampling current-mixing (shunt-shunt) topology.

Calculation:

In the given circuit, the feedback element is R₃

1. As the feedback element is not directly connected to the output node,

it indicates current sampling.

2. Also the feedback element is not connected to the input mode, then it indicates series mixing.

Hence the given circuit has current-series feedback or Series - Series feedback.

Hence option (3) is the correct answer.